1. Field of the Invention
This invention relates to a structure of an air bag cover which has an air bag cover main body rupturable by the expansion of an air bag outward toward an occupant of a vehicle at the time when the vehicle suddenly decelerates, and an insert member embedded in the air bag cover main body by means of insert molding.
2. Description of the Related Art
An air bag apparatus of the type which is installed, for example, in a steering wheel of a vehicle, is provided with an air bag cover made of a resin such as urethane with decorative outer surfaces (hereinafter referred to as "design surfaces"). Between the air bag cover and the base plate Of the air bag apparatus an air bag is accommodated which expands outward toward an occupant of the vehicle when the vehicle suddenly decelerates.
Recently, as shown in FIG. 6, some air bag covers 100 have arch-shaped design surfaces. FIG. 6 shows a cross-sectional view of a box-shaped air bag cover 100 along a plane roughly parallel to the upper wall (not shown) of the air bag cover 100. In such an air bag cover 100, when the upper wall is ruptured, the upper wall is subdivided into two sections which open like a biparting door upward and downward, respectively, as viewed in FIG. 6. Hinge portions 100A act as pivots for the opening movement of the divided sections of the upper wall, and have an arched cross section. Accordingly, hinge portions of insert plate 102 embedded in the air bag cover 100 must be formed so as to have an arched cross section corresponding to that of the hinge portions 100A of the air bag cover 100.
However, when an air bag cover 100 having convexly arched design surfaces is employed, the rigidity of the inset plate 102 must be taken into consideration.
The air bag cover 104 shown in FIG. 7 has been proposed, in which each of the hinge portions 104A is designed to have a convexly arched design surface while having a straight or flat inner surface. In such design, it becomes possible to employ an insert plate 106, embedded in the air bag cover 104, which has a straight cross section at locations corresponding to the hinge portions 104A. Since the thickness P of the hinge portions 104A between the outer surfaces of the insert plate 106 and the corresponding outer surfaces of the hinge portions is large, the rigidity of such portions must also be taken into consideration.
Taking the above into consideration, an air bag cover 108 shown in FIG. 8 is now employed, which has flat design surfaces. This design eliminates the necessity of arching the insert plate 110 at portions corresponding to the hinge portions 108A, and makes the thickness of the hinge portions 108A thinner.
However, in cases where the above-mentioned air bag cover 108 is employed, it becomes impossible to meet the initial requirement for providing the air bag cover 108 with convexly arched design surfaces, whereby the flexibility in designing the design surfaces of the air bag cover is diminished.